Various applications require that a sealed chamber contain a specific molecular species and the species be contained at a specific pressure and quantity. The conditions under which the chamber is constructed may preclude the presence of the species, and introduction of the species after the chamber is constructed and sealed may be difficult or impossible. In one particular application, the waveguide of a chip-scale atomic clock contains a vapor of selected molecular composition. The molecules may be reactive and accordingly, the molecules may not be present during fabrication of the waveguide.